In recent years, due to a high integration of semiconductor device, a demand for flatness of a semiconductor silicon wafer used for it becomes more and more strict. The flatness is needed to an area near the edge of the wafer to improve a yield of semiconductor chip.
The final shape of a silicon wafer depends on a mirror polishing process that is the last step. In particular, the silicon wafer having a diameter of 300 mm is subjected to first polishing by a double-side polishing in order to satisfy a strict specification of the flatness and then second polishing of a single-side surface and final polishing are performed in order to improve a scratch on the surface and surface roughness. The second polishing of a single-side surface and the final polishing are required to maintain the flatness made by the first polishing of double-side surfaces and to make the wafer surface a perfect mirror surface in which there exist no defects such as a scratch on the surface.
As shown in FIG. 8, a common single-side polishing apparatus comprises a turn table 83 onto which a polishing pad 82 is attached, a polishing agent supply mechanism 84, a polishing head 85 and the like. The polishing apparatus 81 polishes a work W by holding the work W with the polishing head 85, supplying the polishing agent 86 to the polishing pad 82 from the polishing agent supply mechanism 84, rotating the turn table 83 and the polishing head 85, respectively, and bringing the surface of the work W into sliding contact with the polishing pad 82.
As a method for holding the work on the polishing head, for example, there is a method of attaching the work onto a flat disk-shaped plate through an adhesive such as a wax and the like. Other than that, as shown in FIG. 9, there is a method in which a elastic film that is referred as a backing film 93 is attached to a work holding plate 92 to hold the work for the purpose of suppressing a transfer of the concavo-convex shape of polishing head body 91 and the work holding plate 92. In addition, there is a so-called rubber chuck method in which a work holding portion is made of a rubber film, a pressurized fluid such as air is poured into a back face of the rubber film, and the rubber film is inflated by a uniform pressure so as to press the work to the polishing pad (See Japanese Patent Application Laid-open (kokai) No. 2002-264005). A polishing head provided with a retainer ring outside the work, the retainer ring which is a means for pressing the polishing pad, is also proposed for the purpose of suppressing sag in an outer peripheral portion of the work to improve the flatness.
An example of structure of the conventional polishing head of a rubber chuck method is schematically shown in FIG. 10(a). The structure is as follows. A rubber film (rubber material) 104a is attached in such a manner that a concave portion of a mid plate 102a provided with the concave portion at its lower face is sealed. A fluid is supplied to a first sealed space portion 103a through a first pressure adjustment mechanism 105a so that a wafer W can be pressed, so-called a rubber chuck structure. Moreover, the mid plate 102a is connected with a polishing head body 101a through an elastic film 106a. A fluid is supplied to a second sealed space portion 107a, which is sealed with the elastic film 106a, through a second pressure adjustment mechanism 108a so that the mid plate 102a can be pressured. An annular guide ring 109a is connected with the polishing head body 101a to hold the wafer during polishing process so that the guide ring is arranged outside the wafer W. Moreover, there is a method of pressuring only by the rubber film 104b without a pressuring mechanism of the mid plate 102b as shown in FIG. 10(b). Furthermore, as shown in FIG. 10(c) a polishing head provided with a retainer ring, which presses the polishing pad instead of the guide ring, is also proposed for the purpose of suppressing sag in the outer peripheral portion of the work. The retainer ring 109c presses the polishing pad with being supplied a fluid to a third sealed space portion 111c, which is sealed with the elastic film 110c, through a third pressure adjustment mechanism 112c. 
In the case of a structure in which the rubber film is provided in tension at an opening end portion of the concave portion of a mid plate as shown in FIG. 10(a), (b), there is a problem that stiffness of the rubber film near the opening end portion substantially becomes high under the influence of the tension, pressure applied on the outer peripheral portion of the work becomes high, and thereby an outer peripheral sag occurs. Moreover, a method in which a position of a holding portion P (the opening end portion) of the rubber film is raised against the work to decrease pressure of the outer peripheral portion so that the outer peripheral sag is suppressed as shown in FIG. 10(d) is proposed (See Japanese Patent Application Laid-open (kokai) No. 2002-264005, for example). However, there occurs a problem that trying to improve the outer peripheral sag of the wafer results in forming a shape raised on the outer periphery of the wafer, uniformity deteriorates, the shape does not stabilize under the influence of variability of the tension when the rubber film is provided in tension, and the like. The method in which a retainer ring is provided outside the work to directly press the polishing pad so that the outer peripheral sag is suppressed as shown in FIG. 10(c) is also proposed. However, since a retainer material is also polished, there occurs a problem that a scratch is generated on a surface of the work under the influence of generation of dust and the like from that, a polishing agent is not sufficiently supplied to the work surface because it presses the polishing pad, and thus a decrease in polishing rate is caused and the like.